The present invention relates generally to electrical capacitive circuits which may be variably adjusted or trimmed, and more particularly to enhancing the quality factor (Q) of such capacitive circuits. It is anticipated that a primary application of the present invention will be in combination with electronically settable instances of such capacitive circuits which often have Q which is lower than desirable due to intrinsic resistance.
Many electronic circuits require adjustment of particular parameters, either during initial assembly or later, as changing conditions or deterioration due to age, etc. affect such parameters. Of present interest is capacitance, since setting capacitance is critically important to insure the proper operation of a wide variety of electrical circuits and circuit-based processes which are in wide use. Capacitance may, of course, be set by making an initial choice of or performing replacement with a fixed-value component. This will generally not be dealt with here, but it should be kept in mind that substituting an adjustable capacitor for a fixed-value one may be useful in many situations.
Capacitors which may be variably adjusted are desirable, or even critically necessary, in many electronic circuits. For example, computers, clocks, radios, televisions, garage door openers, and a myriad of other electronic devices all have one or more internal circuits that require correct capacitance value adjustment. Adjusting capacitance to achieve such values can be done at the time of initial assembly, where it is almost always necessary, or it can be done later by readjusting back to the original value or by changing to an entirely new capacitance value, as appropriate.
For purposes of the following discussion, the operation of adjusting a variable capacitance device is collectively termed xe2x80x9ctrimming,xe2x80x9d regardless of the specific device type. Further, the operation of setting a variable capacitance device within a more general circuit is collectively termed xe2x80x9ctuning.xe2x80x9d It should be appreciated that these definitions are broad and widely encompassing ones. For example, while some might consider tuning to be proprietary to operations in particular industry segments, such as the adjustment of radio frequency oscillators and amplifiers, and to therefore not also encompass operations like setting delay circuits or stabilizing digital memories, such restrictive interpretation is not intended and is not appropriate here.
Tuning requires determining what capacitance value will be correct in a circuit, and then providing and trimming a xe2x80x9ctuning capacitorxe2x80x9d in the circuit to that value. Unfortunately, this can be an expensive, time-consuming, and error prone process.
FIG. 1 (background art) is a diagram depicting a general circuit 1 requiring capacitive tuning. A conventional analog tuning device 2 is connected to the general circuit 1 for this purpose. The analog tuning device 2 may be as simple as a standard adjustable capacitor, or it may be a complex assembly used to achieve the net effect of adjusting capacitance.
FIG. 2 (background art) depicts the usual choice made today for the analog tuning device 2, a variable analog capacitor 3. Presently the most commonly used variable analog capacitors 3 are mechanical in nature. For example, one type includes several semicircular plates which are rotated relative to other semicircular plates which are fixed. The amount by which the respective plates overlap then determines the capacitance. If the rotating plates do not overlap the fixed plates at all, the capacitance is nominally zero, and if the plates completely overlap, the maximum capacitance of the device is reached. Adjusting to any capacitance between zero and the maximum is thus possible.
Unfortunately, this type of variable analog capacitor 3 has a number of disadvantages. For example, the mechanical relationships of the plates are much subject to undesirable change by shock and vibration. Other, more subtle, influences on the net capacitance are variation in temperature, pressure, and humidity. The dielectric, or inter-plate medium, in such capacitors is often air or another gas, and must usually be kept relatively contained and uncontaminated. Liquid or gel filled adjustable capacitors are also possible but are uncommon, due to concerns such as fill leakage past the seals around adjustment mechanism shafts, etc. Entirely solid dielectrics are not possible, since the plates must permit movement. In sum, variable analog capacitors 3 have numerous inherent characteristics that make them unreliable and failure prone.
Another consideration is utility. To adjust or to readjust the variable analog capacitor 3 requires physical access to perform the mechanical adjustment operation, which is typically rotation. In large and complex systems other components and entire other systems may obscure physical access. In small and compact systems such access may also be difficult, and can even subject surrounding components to potential damage. In hazardous locations, such as the ocean""s depths, physical access can be quite difficult, and in remote locations, such as those visited by space probes, physical access can be outright impossible.
When used for tuning, devices such as the variable analog capacitor 3 often introduce another limitation. For many tuned circuits the quality factor (Q) is very important. For example, the higher the Q value is in oscillator tuning, the more precise and narrow the bandwidth of the signal produced. Similarly, the higher the Q value is in amplifier tuning, the narrower the bandwidth selection that is possible.
Unfortunately, while obtaining a low Q value usually is not difficult, obtaining a high one often can be. Almost all practical electrical devices have inherent characteristics which affect the attainable values for Q in circuits using those devices. For example, the leads 4 to the variable analog capacitor 3FIG. 2 have some small but appreciable resistance. Such xe2x80x9cintrinsic resistancexe2x80x9d can come from many sources. If the variable analog capacitor 3 is mounted on a printed circuit (PC) board and thereon connected to an integrated circuit (IC), the PC board traces and the IC pins or terminals will contribute to the intrinsic resistance. If the electrical connection to the variable analog capacitor 3 passes through one or more switches, the resistances of physical switch contacts or solid state switch materials will also contribute to the intrinsic resistance.
The variable analog capacitor 3 has been used as an example here, but even entirely solid state trimable capacitors, as will be discussed presently, can be severely limited with respect to their attainable Q, due to the presence of severe intrinsic resistance. Accordingly, what is further needed is a structure that will enhance the quality factor (Q) of a capacitive element.
Accordingly, it is an object of the present invention to provide a structure to enhance the quality factor (Q) value of a capacitive element.
Another object of the invention is to provide a structure to enhance the Q value of a simple capacitive element when it is necessary or desirable to use the element in series with resistances which lower the Q value.
And another object of the invention is to provide a structure to enhance the Q value of a simple capacitive element when it is necessary or desirable to use the element in series with switch systems having intrinsic resistance.
Briefly, one preferred embodiment of the present invention is a quality factor enhancing structure for a capacitive circuit. The capacitive circuit exhibits an internal resistance, which may be treated as one or more series resistances, as well as an internal capacitance between a first and second connection nodes. The present embodiment includes a first capacitor having its first pole connected to a first terminal node and its second pole connected to the first connection node of the capacitive circuit. The second terminal node is connected to the second connection node of the capacitive circuit, thus presenting the first capacitor in parallel across the capacitive circuit. A second capacitor is further included, having its first pole connected to the first connection node of the capacitive circuit and its second pole connected to a second terminal node, thus presenting the second capacitor in series with the capacitive circuit.
An advantage of the present invention is that it provides a structure of potentially small size, and thus one highly suitable for easy incorporation into more general electronic circuits.
Another advantage of the invention is that, when the workpiece-like capacitive circuit is also small and suitable for such use, the structure and the capacitive circuit together may be assembled into highly desirable packaging types such as integrated circuits and sealed or encapsulated modules.
Another advantage of the invention is that it has a low component count, reducing material usage and manufacturing steps, and accordingly making the structure quite economical.
And another advantage of the invention is that it has a low terminal count, and thus need not add any terminals or pins to those that an underlying conventional capacitive circuit would necessarily already have.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in the several figures of the drawings.